1. Field of the Invention
The present invention relates to an image forming optical system and an electronic image pickup apparatus equipped with the same.
2. Description of the Related Art
In recent years, an image pickup apparatus such as a digital camera has become popular, replacing a silver-salt film camera. In a digital camera, an object is captured by using a solid image pickup element such as a CCD (charge coupled device) and a CMOS (complementary metal oxide semiconductor). It is desirable that an image pickup lens which is to be used in such image pickup apparatus is a zoom lens (image forming lens) with a high zoom ratio.
Moreover, in such image pickup lens, it is desirable that an aberration related to an image forming performance (such as a spherical aberration and a coma aberration) with single color has been corrected favorably. Furthermore, it is desirable that a correction of the chromatic aberration related to color spreading and resolution of image is corrected sufficiently.
On the other hand, shortening of an overall length of lens (overall optical length) is desired. Incidentally, as shortening of the overall length of lens and small-sizing of an optical system as a whole are facilitated, various aberrations, particularly the chromatic aberration occur substantially, and the image forming performance tends to be degraded. Particularly, in a zoom lens having a long focal length at a telephoto end with a high zoom ratio, as a correction of the chromatic aberration, a reduction of a secondary spectrum in addition to a primary achromatism has been sought.
As a method for reducing an occurrence of such chromatic aberration, a method in which an optical material having an abnormal partial dispersion ratio is to be used, has been known (Japanese Patent Application Laid-open Publication Nos. 2007-163964, 2006-349947, and 2007-298555).
Moreover, it is desirable that a zoom lens to be used in the image pickup apparatus has a predetermined zoom ratio, and a wide angle end is a wide angle of field, and has a brighter and improved performance. For improving the performance of the zoom lens, it is necessary to correct the chromatic aberration favorably over the entire zoom range.